You call it Jell-O, I call it Jelly. It's that US/British pronunciation difference again. (What Americans call jelly, the Brits call jam). Whatever your preference, mine is for that wibbly wobbly dessert that has come roaring back into fashion, led by kitchen alchemists like Heston Blumenthal distilling intense flavors into translucent confections.

Jellies can be sweet or savory. They can be architectural, decanted from complex molds, or plain, put to set in a simple glass. But made properly, not too stiff, not too liquid, they're spectacular. From the classic French chaud-froid concotions of cold meats or fish compositions under a glaze of jelly, to vegetable terrines in aspic, consommé, and the Italian vanilla-cream jelly panna cotta, these dishes require a agent to stiffen them.

So what makes them gel?

19th century European cooks boiled up calves' and pigs' feet to extract their gelatin. Today's gelatin (with a final 'e' if you're British) is made by boiling down the bones, skins, tendons and glutinous parts of animals. Which is why, during Britain's BSE crisis, production in the UK was stopped. It comes in powder or leaf form as a brittle, transparent, slightly yellow sheet with an indefinable whiff of taste. Anything made with gelatin has a brilliant capacity to melt slowly on the tongue.

Agar-Agar is a vegetarian and vegan version, derived from seaweed. By contrast, a jelly made from this doesn't melt, it almost snaps, almost crunches between the teeth. It's easy to find in Asian markets where it's sold in powdered form, in noodle-type strings or in long blocks, flat white in color. Other alternatives to animal gelatin include powder made from the root of the Kudzu, that dreadful creeping bush vine invader to our countryside, carrageenan (more seaweed) or guar gum.

I find sheet gelatine easier to use than powdered gelatine. Just lay the number of leaves required by the recipe onto water, allow it to soften for five minutes then gently warm the mix to fully dissolve the gelatine.